1. Field of the Invention
This invention relates to an apparatus for controlling a multiplication factor of an optical receiving means and controlling a gain of amplifying means for electrical signals corresponding to the received optical signals. More specifically, the invention relates to an automatic multiplication factor and gain control apparatus which has application in a repeater for an optical signal transmission apparatus, etc.
2. Description of the Related Art
In an optical receiving system, such as an optical repeater etc. which are used in an optical communication apparatus, a photoelectric converting circuit having an optical receiving element, such as an avalanche photodiode, has been used. An automatic gain control apparatus is used for transmitting a received optical signal without deteriorated amplitude. The apparatus controls the multiplication factor of the optical receiving element and the gain of a circuit for amplifying electrical signals corresponding to the received optical signals.
As shown in FIG. 6, a well-known apparatus has an optical receiving element 51, such as an avalanche photodiode (APD), which converts and amplifies received optical signals into electrical signals with an adjustable multiplication factor. The multiplication factor of optical receiving element 51 is controlled by a change of a bias signal applied thereto. A variable gain amplifier 52 amplifies the converted electrical signals from optical receiving element 51. The amplified signals from variable gain amplifier 52 are transmitted to a transmitting system and are also supplied to a peak detector 53 which detects peak levels of the amplified signals. A comparator 54 compares an output voltage corresponding to the peak level with a predetermined voltage corresponding to the peak level with a predetermined voltage from a voltage source 55. When the output voltage from peak detector 53 is less than the predetermined voltage, a voltage control circuit 56 generates a bias voltage having a level corresponding to an output signal from comparator 54. The multiplication factor of optical receiving element 51 is controlled by this bias voltage. The output signal from comparator 54 also controls the gain of amplifier 52. By appropriate control of the multiplication factor element 51 and the gain of amplifier 52, an output amplitude from amplifier 52 is kept constant regardless of optical input levels in optical receiving element 51. When the received optical signal has a low level corresponding to the output voltage from peak detector 54, which is lower than the predetermined voltage, the gain of variable gain amplifier 52 is controlled to be constant by a control signal from comparator 54. When the level of the optical signal changes, the output amplitude from amplifier 52 is usually controlled to be constant by changing the multiplication factor of optical receiving element 51. However, when the received optical signal has a high level, as determined by the output voltage from peak detector 54 being greater than the predetermined voltage, the output amplitude from amplifier 52 is kept constant by changing the gain of amplifier 52 without changing of the multiplication factor of element 51.
In FIG. 7, a continuous line shows a relation between the level of the received optical signal and the multiplication factor in the optical receiving element. A dashed line shows a relation between the level of the received optical signal in the optical receiving element and the gain of the variable gain amplifier. As shown in FIG. 7, when the level of the received optical signal is low, the gain is constant. As the level of the received optical signal rises, the multiplication factor becomes lower and eventually stabilizes at a constant minimum level. Also, the gain becomes low. Then, the output amplitude from variable gain amplifier 52 is kept constant.
Optical receiving element 51, when embodied as an avalanche photodiode, generates noise with its value dependent on the multiplication factor. Also, the avalanche photodiode has an optimum multiplication factor in which a signal to noise ratio becomes maximum. FIG. 8 shows a relation between the level of the received optical signal and the signal to noise ratio, in a receiving system of an optical repeater, which depends on ways of controlling of the multiplication factor. A continuous line shows a case in which the multiplication factor is controlled so that the signal to noise ratio becomes maximum. A dashed line shows a case in which the multiplication factor is kept constant. A chain line shows a case in which the multiplication factor is controlled and variable. As shown in FIG. 8, when the level of the received optical signal increases, the signal to noise ratio in the constant multiplication factor is better than that in the variable multiplication factor. However, when the multiplication factor is kept constant, a dynamic range of the level of the received optical signal is restricted and deteriorates. Thus, it is difficult to secure a constant output amplitude from the variable gain amplifier with a wide dynamic range of the received optical signals.